The disclosure concerns a roller for transferring a print image or a toner layer onto another element in a printer or copier, wherein the roller has a base body and a first elastic layer applied onto the base body.
In printers or copiers, print images and toner layers are often transferred from one roller to another roller. The transfer of the print image onto the actual printing substrate also often takes place with the aid of rollers.
Given the transfer of the toner layer or of the print image between two rollers it is typical that one of the two rollers has an inelastic surface and the other roller is coated with an elastomer. The transfer of the print image onto the printing substrate also normally takes place with an elastomer-coated transfer roller past which the printing substrate web is directed. A hard, inelastic roller is in particular arranged in turn on the side of the printing substrate web that is opposite the elastomer-coated transfer roller. Due to the elastomer coating, a pressure profile develops within the contact zone upon transfer of the print image or of the toner layer. Due to the elasticity of the elastic layer, this pressure profile may be made uniform. A compensating effect with regard to mechanical tolerances and deformations thereby also takes place. A better print quality is thus achieved.
Upon pressing together the roller with the elastic layer and a hard roller without an elastic layer, the elastic layer is deformed. This deformation includes a radial component and a tangential component, wherein the desired adaptation to tolerances and deformations takes place via the radial component. The tangential component depends on the elastic properties of the roller coatings. The deformation may thereby lead to an enlarged or reduced contact zone between the rollers. The greater the force with which the two rollers are pressed together, the stronger the tangential deformations. Due to the tangential deformations of the contact zone between the two rollers, the surface velocity of the coated roller increases relative to the hard roller in the region of the contact. A relative velocity between the two rollers—which is unwanted in a printing process—is hereby created that leads to a negative effect on the print quality. This local variation of the surface velocity is generally designated as a conveying behavior.
What is particularly problematic with the conveying behavior is that this is not necessarily equally pronounced over the entire contact zone, but rather may be of different magnitude at different locations depending on the distance from the edge of the roller. This has the consequence that a countermeasure purely via variation of the drive velocities of the rollers could never entirely compensate the conveying behavior for all locations, and thus negative effects on the print quality due to the conveying behavior still take place at least at some locations.
Moreover, the conveying behavior is also different as viewed in the tangential direction of the contact zone, such that a corresponding countermeasure is not possible.
To minimize the conveying behavior, printing blankets are known from offset printing, which printing blankets are comprised of multiple layers of elastomers and rigid fabric. The application of such printing blankets for transfer printing rollers is not possible since a seamless roller coating is required.
From the document WO 2007/077053 A 1, a roller coating is known with the aid of which a defined compressibility of the material should be achieved via introduction of voids. A reinforcement hereby takes place via a grid. It is hereby problematic that a homogeneous electric field between the rollers is necessary for transfer printing, which would be prevented by such a grid.
The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings.